"Heartlines" is a song written and recorded by New Zealand music duo Broods for their second studio album, Conscious (2016). The song was co-written by fellow New Zealand musician Lorde and the track's producer, Joel Little. It was first released to digital retailers on 10 June 2016 as the first promotional single from the album. "Heartlines" was serviced to American hot adult contemporary radio through Capitol Records on 16 January 2017 as the second official single from Conscious.

Upon its initial release, "Heartlines" entered the New Zealand Hitseekers chart at number 3. The song generated media attention for its accompanying music video, which stars Rose McIver and was created in collaboration with Microsoft to demonstrate the capabilities of its bio-interactive Microsoft Band technology.

"Heartlines" is a midtempo synthpop song written by Georgia and Caleb Nott of Broods with Lorde and Joel Little. The song employs a "pulsing" electronic beat and a "slow, tantalizing build," while Georgia's vocals have been described as "smokey" and similar in cadence to co-writer Lorde. Lyrically, "Heartlines" explores the "uneasy emotional territory" that goes along with a long-distance relationship. "It's about the person you're into living on the other side of the world," Georgia told Rolling Stone Australia, "Every time you see them it's amazing but it's never enough to fully invest yourself."

Tim Forster of Exclaim! cited "Heartlines" as a "pleasing moment" from Conscious in a mixed review of the album, writing, ""Heartlines" ... smoothly [executes] an ambiguous neither-major-nor-minor sentiment." The staff of music blog Popmatters gave the song an average rating of 6.25, based on four reviews. "Broods' Georgia and Caleb Nott may not be the most distinctive dance-rock architects in the contemporary arena," writes Pryor Stroud in one of the more complimentary reviews, "but they know how to engineer an unimpeachable synthpop anthem. ... With "Heartlines", [sic] they continue to showcase this deft sense of sonic craftsmanship."

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